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"Cecil Moore" wrote
Take your NEC helical model and adjust the frequency to approximately double the resonant frequency and take a look at the current distribution. I have already done an illustration based on the currents in the NEC comparison posted earlier, showing a helix and a linear monopole each about 6 degrees in aperture (link below). This link shows that even though the length of wire used in the helix is 3.14 X the length used in the linear monopole, the current distribution along their apertures essentially is the same, as will be the directivity and radiation pattern of both versions. This same equivalence would apply to the current distribution, directivity and pattern of a linear, 1/4-WL monopole and a helically-wound monopole that was 1/4-WL in aperture, but contained 1/2-WL of coiled wire. http://i62.photobucket.com/albums/h8...le_Current.gif |
#2
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On Mar 30, 10:28*am, "Richard Fry" wrote:
I have already done an illustration based on the currents in the NEC comparison posted earlier, showing a helix and a linear monopole each about 6 degrees in aperture (link below). What we have here is a failure to communicate. Please forget about your previous posting. We are not talking about 6 deg. electrically short helicals. We are talking about comparing an 180 degree electrically long monopole to a 90 degree long RESONANT monopole. Here's how to accomplish what we are talking about: 1. Wind a helical that is 90 degrees long, i.e. the feedpoint impedance is R1+j0. That helical is 1/4WL long electrically and resonant. It may be ~1/8WL (45 deg) long physically. 2. Now increase the frequency until the helical is 180 degrees long electrically. At something like double the frequency, it will be 1/2WL long electrically and the feedpoint impedance will be R2+j0 where R2R1. It may be ~1/4WL long physically. John said his 180 degree helical outperformed his resonant 90 degree helical. His statement has nothing to do with electrically short helical monopoles because they are resonant. The current maximum for a 90 degree resonant helical will be at the base feedpoint just as it is for a 90 degree stub. The current maximum for a 180 degree helical will be halfway up the antenna just as it is halfway up a 180 degree stub. -- 73, Cecil, w5dxp.com |
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